Electricity meters measure electrical energy consumed by a facility. Electrical utility service providers, or simply utilities, employ electricity meters to gather energy consumption data for customer billing and other purposes. Common forms of electricity meters are electronic-based meters and electromechanical meters. Electromechanical meters employ rotating disks driven by electromagnetic fields in order to detect energy usage. By contrast, an electronic meter measures electrical energy consumption by sampling scaled-down versions of the voltage and current waveforms on the power lines of a facility and the performing energy consumption calculations using the sampled waveforms.
More specifically, electronic meters employ sensors that “scale down” the voltage and current waveforms to generate representative measurement signals. The measurement signals are then passed one or more A/D converters that sample the measurement signal to generate the “digital” voltage and current waveforms (i.e. series of digital samples). A processing circuit then performs mathematic operations on the digital voltage and current waveforms to calculate electrical energy consumption.
Electricity meters must have the ability to accurately measure currents over a broad dynamic range. While the RMS line voltage seldom varies, the RMS line current can change significantly as energy consumption by the customer changes. As a consequence, a meter must accurately measure as little as 1 Amp RMS and as much as 225 Amps RMS. This requires that the A/D converter be accurate over that same range.
This Accuracy requirement presents a problem because A/D converters inherently have less resolution when measuring quantities that are small with respect to the AID converter's range. For example, if an A/D converter having 256 discrete levels measures from 0 to 225 amps, then a small current, for example two amps RMS, will be represented by only 2 or 3 of the available discrete quantization levels. As a consequence, the digitized waveform will not have adequate resolution. By contrast, a 200 amp current waveform in the same A/D converter will be represented by well over 200 discrete quantization levels, thereby providing a fairly smooth and accurate representation of the waveform. Thus, it is difficult to obtain accurate digital representations over a broad range of current magnitudes.
Moreover, the inherent inaccuracies that exist in many A/D converters can be exaggerated when attempting to digitize signals having a peak value that is well below the effective scale of the converter. One such inherent inaccuracy of A/D converters is quantization step error. A quantization error in a converter that causes a relatively insignificant error when digitizing large input signals can cause a relatively substantial error when digitizing very small input signals.
As a consequence, to ensure accuracy over the entire dynamic range of current levels in an electricity meter, the A/D converter must be chosen such that it is highly accurate at low RMS current levels as well as at high RMS current levels. To this end, an A/D converter may include sufficient quantization step levels such that even very low current waveforms cover a number of quantization levels. Thus, it may be necessary to use an extremely high resolution A/D converter in order to ensure accuracy at low level currents. Unfortunately, increasing the number of the quantization levels of the A/D converter adds cost.
Moreover, because the excess quantization levels are not necessary at medium and high RMS current levels, the additional cost is not always justified. Nevertheless, the meter must satisfy industry and government accuracy standards over its entire useful range, including the low RMS current range.
There is a need, therefore, for an electricity meter that performs energy calculations uses digital signals, achieves accuracy over a wide range of currents, and does not suffer the cost disadvantages due to increase accuracy requirements in the A/D converter.